1. Field of the Invention
The present invention relates to a method for reading optic disc data, and more particularly, to a method for sampling optic disc data.
2. Description of the Related Art
Among different types of storage media, the optical disc is commonly used in various fields due to its characteristic of high capacity, low cost, and portability. It is applied in storing data, listening music, and watching movies. In order to adapt to the characteristic of optical disc and to increase the reliability of the optical disc data reading, during the data is being stored into the optical disc, the current burning status in optical disc is changed when the data value is 1, and the current burning status in optical disc is not changed when the data value is 0.
During the data is being stored into the optical disc, an eight to fourteen modulation (EFM) process and a Reed-Soloman code modulation process are performed on the data, and the sequence of data storing is interleaved. The EFM process converts 8-bit data into a 14-bit data for in Compact Disks (CDs) and converting 8-bit data into a 16-bit data for Digital Versatile Discs (DVDs), and the EFM processed data is stored into the optical disc.
In order to read data from the optical disc, it is required to sense the laser light reflected by the optical disc to obtain a radio frequency data (RF DATA) first, and then based on the RF DATA to generate a digital data signal DSEFM and a clock signal EFMCLK. The EFM data decoding are applied further onto these two data signals so as to perform a subsequent optical disc decoding process.
Since the optical disc provides a high density for data storage, and the reading speed of the current optical disc drive is higher now, many techniques for improving the reading reliability (e.g. EFM technique mentioned above) have to be involved in order to accurately read the content of the optical disc. In addition, for every a predetermined length of the data stored in the optical disc, a sync pattern is inserted into the data for accurately reading the optical disc data. The session of the stored data with a predetermined length plus the sync pattern mentioned above is referred to as a frame.
However, the data read from the optical disc is a serial signal. In order to accurately read the serial signal, it is required to divide the serial signal based on an accurate clock signal. If the timing of dividing the serial signal is not correct, the data signal DSEFM obtained is misread. In the prior art, a timing for detecting a sync pattern is used as a base for dividing data, and a counter is used to issue a data sampling tag signal when the clock signal EFMCLK counted by the counter reaches the length of the EFM data unit. As a result, the next stage circuit can accurately sample the data signal DSEFM according to the timing of the data sampling tag signal.
FIG. 1A is a schematic block diagram of a conventional optical disc data sampling apparatus. FIG. 1B is a timing diagram of the signals shown in the circuitry of FIG. 1A. Referring to both FIGS. 1A and 1B, a sync detection circuitry 110 receives a clock signal EFMCLK and a data signal DSEFM, and detects a sync pattern “sync” from the data signal DSEFM based on the clock signal EFMCLK. Once the sync detection circuitry 110 detects the sync pattern “sync”, a sync tag signal “SyncTag” is issued. A counter 120 counts the number based on a timing (e.g. a timing of the clock signal EFMCLK) and generates a counting result “cnt”. The counter 120 resets the counting result “cnt” based on the sync tag signal “SyncTag” or a reset signal “reset” and recounts the number again. A comparator 130 receives the counting result “cnt” and compares it with a predetermined value “n”. Once the counting result “cnt” is accumulated to the predetermined value “n” , the comparator 130 issues a data sampling tag signal “DataTag” and the reset signal “reset”.
FIG. 1B schematically illustrates the relationship among the signals mentioned above. Under a normal situation, the counter 120 of the conventional optical disc data sampling apparatus automatically counts the number and regularly generates a data sampling tag signal “DataTag_N” (i.e. the data sampling tag signal “DataTag” shown in FIG. 1A). Ideally, the timing spacing of two neighboring data sampling tag signals “DataTag_N” should match to the timing of a single record data in the data signal DSEFM. However, errors occur in the timing of the physical data sampling tag signal “DataTag_N”, and the errors may be a lead or a lag error. As the reading data “Data” is increasing, the timing error of the data sampling tag signal “DataTag_N” gradually accumulates, which causes an error on the result of the dividing and sampling of the data signal DSEFM. The data sampling tag signal “DataTag_N” shown in the diagram is an example of a timing lag for explaining the error mentioned above, for example, the signal after the signal “An” is mildly lagged. In order to solve this problem, the prior art uses a sync tag signal “SyncTag” to control the counter 120 for recounting, thus the timing of the data sampling tag signal “DataTag_N” has to be tuned at the right timing. For example, the signal “Bn” shown in the diagram is a signal tuned by the signal “STb” to the right timing.
However, it is well known that the optical disc which is a round plate make of a plastic material is easily scratched. When reading a defective area caused by a scratch or other reason, the sync pattern “sync” cannot be normally detected (i.e. it is not possible to generate the sync tag signal “SyncTag” at the right timing based on the assumption of the sync tag signal “STb” is has not happened). In the prior art, during the reading of the defective area, the sync tag signal “SyncTag” generated by the previous good data area (e.g. the sync tag signal “STa” as shown in the diagram) is used as a base timing to control the counter to continuously count the number, and to continuously generate a data sampling tag signal “DataTag_W” accordingly until next sync tag signal “SyncTag” is appeared (e.g. the sync tag signal “STc” as shown in the diagram). After comparing the data sampling tag signal “DataTag_N” generated under a normal situation with the data sampling tag signal “DataTag_W” which is not accurately tuned in time, it is known that the timing error of the signal “Bw” is continuously accumulated due to the fact that the signal “STb” does not appeared in time. Finally, the signal “Cw” cannot accurately divide the data “Data” (originally, the signal “Cw” should have the same timing as the signal “Cn” has).
In summary, in the prior art, when sampling the data read from the optical disc, the clocks are counted for providing a data sampling tag signal “DataTag” at a right time, and the sync tag signal “SyncTag” is used to correct the timing of the data sampling tag signal “DataTag” in time. When reading the defective area caused by a scratch or other reason, usually it is not possible to generate the sync tag signal “SyncTag” at the right time. In such a case, the prior art uses a sync tag signal “SyncTag” generated by the previous good data area as its base timing to control the counter to continuously count the number, and to continuously generate the data sampling tag signal “DataTag” accordingly until next sync tag signal “SyncTag” appears.